The transmission of data in polarization division multiplex whereby two optical data signals have the same wavelength with orthogonal polarizations is a highly promising method of doubling transmission capacity without having to place more exacting requirements on the transmission link or signal-to-noise-ratio.
However, a disadvantage of polarization division multiplex is susceptibility to polarization mode dispersion (PMD) which results in mutual interference between the transmission channels. Although the effect of PMD can be reduced by PMD compensation measures, compensation is required for each channel of a wavelength division multiplex system; it is also complex/costly and does not always produce the desired results. The use of PMD-optimized fibers likewise provides an improvement, but is only possible for new networks.